The invention relates to an absorption refrigeration machine according to the Platen-Munters system, with a generator for evaporating a refrigerant in a solvent, a solvent separator in which the separation of the solvent from the refrigerant is performed, a condenser for liquefying the refrigerant, an evaporator in which the refrigerant is evaporated using a dry gas while being cooled, optionally with a first gas heat exchanger, and an absorber in which the evaporated refrigerant is added to the depleted mixture of refrigerant and solvent which is then again evaporated in the generator.
To operate a known absorption refrigeration machine according to the Platen-Munters system it is necessary to have a heat source with a temperature far in excess of 100 degrees Celsius. At temperatures of 100 degrees Celsius and below the efficiency tends towards zero. Existing heat sources with low temperatures such as hot water from industrial waste heat system such as long-distance heating, solar heating or the like are not suitable for the conventional designs of said absorption refrigeration machines because the required high temperatures can generally not be achieved.
It is therefore an object of the present invention to provide an absorption refrigeration machine of the kind mentioned above with which it is possible to achieve a high efficiency even at relatively low temperatures, preferably at approx. 75 degrees Celsius.
This is achieved in accordance with the invention in such a way that the output of the evaporator or the output of the optional first gas heat exchanger which is arranged downstream of the evaporator and the output of the generator open into a bypass leading into the absorber, with the mixture consisting of evaporated refrigerant and dry gas which arrives from the evaporator via the first gas heat exchanger being transferred to the output of the generator and from there into the bypass where the gas mixture is brought into contact with the hot, partially degassed solution coming from the generator and withdraws further refrigerant from the same.
The mixture arriving from the evaporator via the first gas heat exchanger is therefore not transferred directly to the absorber, but instead to the output of the generator and from there through the bypass and withdraws refrigerant from the solution arriving from the generator. It is also possible to omit the first gas heat exchanger, so that in this case the mixture is guided from the output of the evaporator to the bypass input. It is possible in both cases to achieve a low solution concentration in the area of the absorber input which is a precondition for a low refrigerating temperature without requiring that the generator be strongly heated. Heat sources of low temperature can therefore be used for the absorption refrigeration machine in accordance with the invention. As a result of the low generator temperature, the quantity of the simultaneously evaporated water is reduced, thus avoiding any losses in efficiency in the evaporator.
In a further development of the invention it may be provided that a second gas heat exchanger is provided whose primary side is disposed between the output of the evaporator or, optionally, the output of the first gas heat exchanger and the input of the bypass and whose secondary side is disposed between the output of the bypass and the input of the absorber, so that the gas mixture arriving from the bypass is cooled. The cooling of the gas mixture allows improving the enrichment of the depleted mixture arriving from the generator.
The bypass allows a low operating temperature, but also causes energy loss. According to a further embodiment of the invention it can be provided that a control valve is arranged between the output of the evaporator and the input of the absorber or between the input and the output of the bypass with which the quantity of the gas can be metered which is rerouted through the bypass, with the part not rerouted flowing directly to the absorber. This ensures a regulation of the bypass effect on the momentarily required temperature reduction of the supplying heat source.
According to a variant of the invention, the control valve can be a straight-through valve which short-circuits the bypass, as a result of which the bypass becomes ineffective when the valve is opened and the bypass can be fully effective when the valve is closed.
According to a further variant of the invention the control valve can be a three-way valve which divides the gas mixture arriving from the evaporator between the flow to the bypass and the flow to the absorber. As a result, the bypass can be set very precisely in its effect.
In order to increase the contact surface between the gas mixture flowing through the bypass and the liquid flowing through the same, the interior wall of the bypass tube can be coated with an ammonia-resistant fiber material, with the ammonia-resistant fiber material preferably being formed by a glass-fiber web which fulfills the requirements placed on a large surface and high durability.
A further feature of the invention can be that in the interior of the bypass tube there is arranged a coil spring resting on the interior wall thereof, with the ammonia-resistant fiber material being tensioned between the interior wall and the coil spring.
It is prevented in this way that the bypass flow cross section decreases for the gas flowing through the bypass.
A particularly high efficiency is achieved in accordance with a further embodiment of the invention in such a way that the refrigerant is formed by ammonia and the solvent by water.
It can further be provided that the bypass is heatable, thus making the bypass temperature adjustable to a value in which the inflowing gas mixture withdraws a very high share of ammonia from the depleted solution.
The invention relates further to a bubble pump for an absorption refrigeration machine with at least one vertical pump tube which is heatable by a liquid or gaseous heat carrier medium and in which a refrigerant solution can be moved upwardly by bubble formation.
The liquid circulation in absorption refrigeration machines is often maintained by so-called xe2x80x9cmammothxe2x80x9d or xe2x80x9cbubble pumpsxe2x80x9d, e.g. in the classical system of Platen-Munters in which water is used as solvent and ammonia as refrigerant. As the energy of a heat source can be taken for the operation of such an absorption refrigeration machine, it is outstandingly suitable for the conversion of solar power into cold. Conventional bubble pumps are not or only badly suited for the heating by heat carrier media with variable temperature as occurs in the winning of solar power.
Such a bubble pump consists of two mutually communicating vessels which are filled with a hydrous ammonia solution. One of these vessels, namely the active part of the pump, is arranged as a small upwardly striving tube which is heated, so that ammonia is released in its interior. The produced gas bubbles drive the solution upward in the narrow tube. In some bubble pumps one can find a small gas collecting vessel in the lower part of the upwardly striving tube in which the tube penetrates from above. The gas collects there before it upwardly pushes the liquid in the tube situated above.
In the two aforementioned types of bubble pumps there is a critical low temperature range in which the gas bubbles will form so slowly that they are too small to fill the entire cross section of the pump tube and will therefore migrate upwardly without entraining any liquid. The ammonia concentration will thus decrease in the pump tube. According to the thermodynamic data of ammonia solution in water, the temperature at which ammonia can be released will then rise. In the case of a slow rise in the temperature of the pump the required minimum temperature will thus rise simultaneously and situations can occur in which the bubble pump will consistently fail to operate because the pump tube only contains water and hardly any more ammonia. The aforementioned gas collecting vessel is provided to reduce this danger. Particularly in the application of solar power a temperature progress over time occasionally still occurs in which bubble pumps with gas collecting vessels will still fail to operate as a result of the aforementioned effect. In the case of an excessively slow starting or cooling process the refrigerant solution can lose too much gas and the bubble pump can thus be put out of operation permanently.
This problem can also occur in gas-heated ammonia absorption refrigeration machines, e.g. when the gas burner is clogged. The pump can only be reactivated when the entire refrigerator has been turned upside down, as a result of which ammonia-rich solution reaches the pump tube again. This process is not viable in large-size refrigerators, which is why large size refrigerating units are usually not equipped with bubble pumps but with electric conveying pumps.
In permanent operation, an optimal degree of refrigerating effect requires a precise dosage of the pump""s output independent of the heating temperature.
It is therefore the object of the present invention to provide a bubble pump of the kind mentioned above with which a failure of the bubble pump in the critical temperature range is avoided and with which a permanent operation of an absorption refrigeration machine is enabled.
This is achieved in accordance with the invention in such a way that the lower end of the at least one pump tube is connected with an oblong heatable pump triggering vessel, which pump triggering vessel is provided with an inlet and an outlet opening and from which the refrigerant solution flowing into the pump tube can be flowed through in a substantially horizontal direction, with the inlet and the outlet opening being arranged in such a way that a gas bubble produced in the pump triggering vessel is held back in the same, with the liquid level of the refrigerant solution in the cold state being below the active pumping range of the pump tube.
Prior to the entrance of the refrigerant solution into the pump tube the same is mostly located in the pump triggering vessel which is heated with a temperature which is always slightly below the actual bubble pump heating temperature. Once the heating temperature rises, a gas bubble is formed in the aforementioned pump triggering vessel, which gas bubble, due to the shape of the vessel, cannot flow off and therefore displaces the solution, so that its liquid level rises up to the currently hot pump tube, so that the pump process is triggered. If the temperature reaches the critical range on the other hand, e.g. where only very little ammonia is released, the pump triggering vessel is already so cool that the ammonia goes into solution, the gas bubble will disappear, and the solution will withdraw from the bubble pump. This is an important difference to the directly heated gas collecting vessels as are known in this connection and into which the pump tube immerses, because no condensation of the gas bubble can occur there as long as the temperature in the active range of the bubble pump still lies above the minimum gas exhalation temperature. A further difference is that the pump triggering vessel is preferably present in the form of a lying tube or any other, surface-enlarging form through which the refrigerant solution flows through the vessel as a thin floor layer below the gas bubble and swirls in the process, thus then enabling the complete reabsorption of the bubble on cooling, because in the case of a non-swirled liquid, as in the conventional gas collecting vessel, a thin layer of the specifically far lighter liquid ammonia will form on the surface which prevents any further solution process. In the bubble pump in accordance with the invention the refrigerant solution is thus automatically removed from the same when the heating temperature decreases to the critical range. On the other hand, the ammonia-water solution can only be located in the active area of the bubble pump at temperatures above the minimum gas exhalation temperature corresponding to the respective system pressure.
It may be provided for in a further embodiment of the invention that the pump triggering vessel is formed by a horizontally arranged hollow cylinder with cover surfaces, with the inlet and outlet opening being provided in the lower zone of the opposite cover surfaces.
As a result, the gas bubble which is produced automatically in case of heating is prevented from escaping through the outlet opening.
According to a further embodiment of the invention it may be provided that the pump triggering vessel is enclosed by a heating jacket through which a liquid or gaseous heat carrier medium can be conducted. This allows fixing the temperature of the pump triggering vessel irrespective of the temperature in the bubble pump, with a continuously slightly lower temperature than the temperature prevailing in the bubble pump being consistently chosen, so that the critical temperature range is reached within the pump triggering vessel already before this and the contracting gas bubble pulls back the refrigerant solution from the pump tube.
It can therefore be provided according to a further embodiment of the invention that the required small temperature difference between the bubble pump and the pump triggering vessel is achieved in such a way that the heating heat carrier medium flows at first through the bubble pump and then through the pump triggering vessel.
According to a further alternative embodiment of the invention, the pump tube can be enclosed by a first concentric heating jacket for flowing through with a liquid or gaseous heat carrier medium and a second concentric heating jacket for a liquid heat carrier medium can be arranged between the pump tube and the first concentric heating jacket, with the liquid level of said liquid heat carrier medium being adjustable within the second concentric heating jacket.
In this way it is possible to adjust the entire thermal resistance of the vessel forming the bubble pump in accordance with the invention to the required heat flow. Furthermore, the heat supply to the pump tube can be regulated independent of the temperature of the heat carrier medium flowing through the first concentric heating jacket.
In absorption refrigeration machines with a generator, an absorber and a condenser it is possible, in a further embodiment of the invention, to arrange a temperature sensor on the connecting tube between the generator and the absorber or on the connecting tube between the generator and the condenser and provide a controller unit with which the pump output can be controlled depending on the temperature measured by the sensor.
Measurements in absorption refrigeration machines and precise calculations prove that the degree of refrigerating effect is only optimal when the pump output of the bubble pump is constant. The pump output fluctuates strongly, however, in the case of variable heating temperature in the case of solar power.
The necessary control of the pump output can occur in such a way that the heat energy supplied to the bubble pump can be controlled independently of the temperature, which is controllable both by a variable contact surface between the heat carrier medium coming from the solar unit and the pump tube of the bubble pump as well as by the change of the heat transmission coefficient at this place.
According to a further embodiment of the invention there is an additional possibility to control the heat transmission coefficient into the bubble pump by changing the flow speed of the heat carrier medium. Since the heat transmission coefficient between a medium and a solid body will rise with the flow speed of this medium and the heat carrier medium needs to be circulated in any case, such a control of the heat transmission coefficient can be coupled in a preferable manner with a control of the flow speed of the heat carrier liquid.
Preferably, the temperature progression between generator and absorber or between generator and condenser of the refrigerating system can be used as a quantity for the pump output, because a larger pump output will shift the range of the higher temperatures closer to the absorber or condenser.
A further feature of the invention can be that the second concentric heating jacket is connected with a gas thermometer by means of whose gas, which expands when heated, the liquid level can be adjusted within the second concentric heating jacket. The gas expanding on heating displaces the liquid from the variable heating jacket about the bubble pump tube which is representative of the variable thermal resistance.
Preferably, the position of the gas thermometer represents the possibility for adjusting the pump output. If the gas thermometer is displaced closer to the absorber or closer to the condenser where the contact surface of the tube is colder, the heating jacket about the bubble pump will enlarge automatically and it will pump more strongly. If, on the other hand, the heating temperature of the pump rises, it will pump quicker and the temperature in the gas thermometer will rise, thus displacing liquid from the heating jacket and braking the pump.